The invention relates to a process and device for the simultaneous manufacturing of a plurality of injection-molded micro parts made from thermoplastic or cross-linking polymers in an injection molding tool having a sprue block with temperable channels for the free-flowing material and having a mold block with a plurality of mold cavities.
For the manufacturing of injection-molded parts made from polymer materials, mold part tools are used which are inserted into injection molding machines. In the case of smaller plastic parts, it is customary to provide several identical mold cavities in one injection-molding tool. Conventionally, the mold cavities are filled with a star-shaped or comb-shaped runner. The material in this runner solidifies together with the mold part and can be reused only partially and only after a regranulation process.
In the case of very small parts with a very low injection volume, as, for example, in the case of electronic components or micro system parts, a multiple of the effective volume is lost because of this simultaneously solidifying runner. Particularly in the case of expensive high-capacity polymer materials, this results in an unacceptable price increase for the product. In the case of problematic additives (such as flame proofing agents), additional costs for the waste disposal must be taken into account.
In order to reduce the manufacturing costs per component, a number of mold cavities per tool is desirable which is as large as possible. However, particularly in the case of modern high-capacity polymer materials, a plurality of mold cavities in a tool has so far resulted in unsolvable filling problems because of unequal flow resistances and flow paths in the case of a central injection by way of the injection nozzle of the injection machine.
From U.S. Pat. No. 5,069,840, a device is known for the counterpressure or oscillation process by means of which the strength-reducing effect of joint lines in the mold part is to be reduced. When there are several sprues per mold part, joint lines are unavoidable. In the case of the suggested process, at least two injection nozzles for each mold cavity are required for filling the mold part. The molding material is simultaneously supplied to the mold cavity and two pressure cylinders by the feeding device. When these are filled with molding material, the connection to the feeding device is closed by valves and the pistons of the pressure cylinders are moved in an alternate fashion. As a result, the molding compound in the mold cavity will oscillate to the point of solidification and the solidification layers are displaced with respect to one another. This increases the strength of the joint lines. The process is therefore suitable only for larger mold parts which require several sprues and the space requirement of the system is too large for very small mold parts.
German Patent Document DE 41 33 435 A1 describes a process in the case of which, by means of plastic waste, mold parts are to be produced which are so large that the extruder capacity is not sufficient for filling the mold cavity in the required time period. In a heated intermediate storage device with a pressure cylinder, an amount of molten material is collected which is sufficient for filling a mold. The smallest mold parts present exactly the opposite problem. The extruder delivers too much material so that an economical manufacturing is possible only if, by the simultaneous manufacturing of a sufficiently large number of mold parts, a minimal amount of material can be conveyed.
It is therefore an object of the present invention to provide a process and a device for manufacturing micro parts without a sprue and with a large number of mold cavities per tool on conventional injection machines.
According to the invention, this object is achieved by a system wherein a single outlet duct of an injection nozzle is assigned to each mold cavity and the material conveyed by a conveyor worm is charged by way of the sprues directly into antechambers having volumes corresponding to assigned mold cavities, the material in this step not reaching the mold cavities, and
wherein, in a second step, injection pistons press the material by way of injection nozzles into the mold cavities, the injection pressure generated by the injection pistons being independent of the conveying pressure of the conveyor worm.
The process according to the invention eliminates the dependence of the filling parameters on the central course of the molding pressure of the injection machine and their unavoidable changes in the course of the sprue runners. By means of the insertion of the injection cylinders according to the invention into the melt flow from the injection machine to the mold cavity, completely new manufacturing possibilities are achieved in polymer processing.
The injection machine first presses the material by way of the sprues into the antechambers of the injection cylinders. The filling of the antechamber presents no problems because its geometrical shape is simple and without any thin-walled contours. The material is then injected by means of the piston of the injection cylinder under a high pressure directly into the mold cavity.
By means of this pressure-related and flow-related uncoupling, the filling and processing parameters of the material can be optimized for each of the mold cavities because the. injection pressure for the mold filling will no longer come centrally from the injection machine but decentrally from the injection cylinders and the material arrives directly in the mold cavity. The pistons of the injection cylinders can be operated individually or jointly, in which case the operating force can be generated hydraulically, pneumatically or mechanically. In the case of a joint operation, it is advantageous to uncouple the individual pistons by means of springs in order to compensate tolerances of all types.
The sprues and the injection cylinders are tempered corresponding to the requirements of the polymer materials to be processed. In the processing of thermoplastics, the sprues and the injection cylinders are tempered and constructed as so-called heating ducts so that the material can be injected into the mold cavity in a liquid state and under high pressure. In contrast, the mold block with the mold cavities is cooled so that the melt is solidified and can be removed from the mold. The processing of cross-linking polymers, such as rubber, requires a reverse tempering where the sprue runner is cooled and the mold block is heated so that the plastic material will not cross-link before it reaches the mold cavity and thus becomes firm. If the tempering is designed correspondingly, the process and the device according to the invention can be used for the tempering of both polymer types.
The injection cylinders may have a very slim construction, whereby a high density per surface unit can be achieved in the arrangement and thus a high number of mold cavities per tool.
The tempering of the injection cylinders may take place individually or several injection cylinders are installed in a heat block which is tempered. The individual tempering results in a precision adjustment for each mold cavity, while, by means of the block tempering, a further densening of the arrangement and thus simpler apparatus-related expenditures can be achieved.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.